Google Wallet for Digital Goods was retired a couple of months back, but they still use it on some Google properties. It's still used for the Chrome Web Store developer fee & I think Android, too. They essentially just made it private.
You might argue that BigTable going through the same thing would be too impactful, but GWDG was quite impactful as well. Businesses that had subscribers paying monthly subscriptions via Google Wallet lost those subscribers.
Are you sure? I'm not a Googler but have been told by other Googlers that Bigtable has essentially been replaced internally (though heard it's still similar). So I wasn't sure how much Bigtable is even used anymore inside of Google.
Of course Google has a whole slew of other storage options optimized for various use cases, but some of these are actually built on top of Bigtable.
You are intentionally forgoing things like coprocessors, and other advanced stuff listed here[1]
[1] https://cloud.google.com/bigtable/docs/hbase-differences
Ultimately, I do love App Engine. It's just the really poor support from Google that is letting down what could be an awesome platform. Neglected bugs, outdated docs, key limitations (eg. no https on naked domains, which looks like it will hopefully be fixed soon) and such.